Is the rock live or base? If live then you want to get your water mixed before adding if base it doesn't matter. The salt mix will get your ph around 8.0 which is fine. You can add buffer per directions but you probably won't have to.

Mobile inverts to avoid are the larger hermits (2+") and larger crabs (4+")

The crabs listed here are all fine. Mostly I'd stick with red/blue leg hermits and get about 10 to start.
For snails the Margarita Snail & Nassarius snail are both great and 10 total of these types would be a good start.

I'd drip acclimate them for 2+ hours before adding to the tank. Also avoid buying any sea stars for now.

I'd wait at least 6 months before adding a sea star. They require very good water conditions to survive long term and are very sensitive to changes in temp/ph/sg. I wouldn't get more then 1 sea star for a 55 gal and only get the Brittle Star which is reef safe. A long 3 hour drip acclimation is best for sea stars before adding to the tank.

Personally I wouldn't add more then 20 hermits or snails to a 55 gal even after it matures. Usually unless you have a ton of lr there isn't enough food to support more then that.

I haven't seen any difference between blue or red leg hermits. The blue leg hermits are prettier IMO but are not any better at scavenging then the red leg hermits.

I'd hold off on any crabs for a couple of months to let your tank mature more. The ones you mentioned are also considered "reef safe" but I'd limit your tank to just one crab.